In recent years, ubiquitous networks using a wireless local and personal areas communication system such as wireless tags, Bluetooth (registered trademark), and ZigBee (registered trademark) have begun to be widely used in the fields of equipment control, traffic, distribution, environmental protection, food industry, agriculture, earthquake monitoring, health care and so on. With the development of applications or services, the number of users of a network is expected to increase in the future. Here, a global ubiquitous network capable of providing several applications or services to a greater number of users and enlarging a service area has been featured.
In such a situation, a network includes a base station connected to a fixed network, and a number of wireless terminals scattered in a wide area. The wireless terminals are directly accommodated in the base station. Further, the wireless terminals in such a network are driven by a battery and are low-power and low-performance terminals having only a minimum number of functions such as data measurement and transmission. Traffic from such wireless terminals to the base station is characterized by (1) a small amount of data and (2) a relatively long transmission interval.
Since a number of such wireless terminals are under one base station, traffic properties have a tendency of a great amount of uplink traffic and an increase of a total amount of traffic. In the network, one base station must accommodate as many wireless terminals as possible in order to collect data from the number of wireless terminals as many as possible. Accordingly, such a network needs a media access control (MAC) protocol capable of achieving high throughput and realizing a short transmission delay time while one base station efficiently accommodates a great number of low-performance wireless terminals.
To satisfy the requirement, a dynamic slot assignment (DSA) method that is a centralized control method is employed as a MAC protocol due to high resource utilization efficiency. The method uses a time division multiple access-time division duplex (TDMA-TDD) as an access scheme. In this method, a base station dynamically assigns slots (bandwidths) according to a request from a wireless terminal.
FIG. 21 shows an example of a configuration of a MAC frame. The MAC frame is divided into two periods: an uplink and a downlink. The downlink period consists of a broadcast period and a demand assignment period, and the uplink consists of a demand assignment period and a random access period.
Channels such as a broadcast control channel (Bch), a frame control channel (Fch), a random access feedback channel (RFch), a control channel (Cch), a data channel (Dch), and a random access channel (Rch) are used in each period in order to transmit and receive data or control information.
Bch is used to inform a wireless terminal of attribute information (e.g., a base station ID and a frame number) of a base station. Fch is used to notify bandwidth assignment information (e.g., a wireless terminal assigned, start point of the assignment, a channel type, and bandwidth amount) of a demand assignment period in which the bandwidth assignment is performed in units of wireless terminal.
RFch is used to notify of random access information (e.g., a random access result of a previous frame, a start position of random access in the present frame, and the number of slot). Cch is used to transmit and receive control information for each wireless terminal, such as a bandwidth request (resource request, RREQ) or automatic repeat request (ARQ). Dch is used to transmit and receive user data. Rch is a channel for random access and is used for the wireless terminal to transmit the bandwidth request (RREQ).
In a DSA method, a random access is mainly employed for a wireless terminal to request a bandwidth because it can accommodate aperiodic, bursty data flexibly and efficiently. FIG. 22 shows an example of an access sequence using the present method. In this example, the base station sequentially transmits Bch, Fch, and RFch from the beginning of a MAC frame. A wireless terminal under the base station can recognize a start position of Rch in the frame and the number of slot by receiving RFch. When the wireless terminal has data to transmit, the wireless terminal transmits bandwidth request information (RREQ) to request a bandwidth for data transmission to the base station using Rch. In this case, the wireless terminal voluntarily determines a back off time, which is a transmission deferred time, based on an exponential back off algorithm in order to avoid collision with other wireless terminals.
The wireless terminal transmits RREQ using the Rch immediately when the back off time is completed (MAC Frame 1 in FIG. 22). When there is a collision with Rch from another wireless terminal, the wireless terminal retransmits RREQ. When the base station correctly receives RREQ, the base station notifies of successful RREQ reception using RFch of a next frame (MAC Frame 2 in FIG. 22), and assigns Dch according to a bandwidth request value from RREQ. Further, in the next frame (MAC Frame 3 in FIG. 22) assigned Dch, the base station assigns Cch for ARQ to inform the wireless terminal of data acknowledgement. Non-Patent Document 1 is known as such a conventional art.
However, in the dynamic slot assignment method using the random access, particularly, when there are a number of wireless terminals under a base station, Rch is highly likely to collide with another Rch, which causes overhead. The overhead is the transmission deferred time caused by the exponential back off algorithm, which deteriorates a throughput characteristic. Accordingly, when a bandwidth request message for a bandwidth used for the event with an immediacy requirement is transmitted from the wireless terminal to the base station, it is difficult to transmit the bandwidth request message with a short delay time.
Now, we assume that a preferential class is requested to be the transmission with a short delay time. Further, to guarantee the preferential class, there is another method which sets the initial back off window (IBW) size of a preferential class to a smaller value rather than that of a non-preferential class. However, since the method uses a random access area for two classes, it is difficult to always guarantee the transmission of the preferential class regardless of the traffic in the non-preferential class. However, since effects of traffic in the non-preferential class cannot be ignored, it is difficult to always guarantee a transmission with a short delay time for the preferential class.    Non-Patent Document 1: Development of 5 GHz bandwidth advanced wireless access (AWA) system-MAC/DCL function-2000 IEICE Society Conference B-5-39 pp. 327.